pV=nRT
How much does all the
air in the room weigh?
How much does all the
water weigh?
What is a hydrogen bond?
What do they do?
What is an ice crystal?
• A collection of organized water
molecules, below 0º C
• Crystal lattice of 4 nearest neighbors,
anchored by H-bonds
• In the atmosphere, grown from vapor,
often but not always by BergeronFindeisen process
• Crystallographic symmetry becomes
expressed macroscopically
• Initiated by several ice nucleation
mechanisms
What is a snowflake?
• A collection of individual ice
crystals falling as a hydrometeor
• Often consists of 5-100 ice
crystals
• Size and crystal numbers depend
on temperature, crystal habits,
and turbulence
• Snowflakes are not made with
scissors
Physical Properties of Ice
c axis
•Density of ice 1h near 0 C: 917kg/m3
•Heat Capacity: 1960 J/kg*K
•Latent Heat of sublimation:
•2.83 x 106 J/kg (to break H-bonds)
•Ice crystal size: several μm to ~5 mm
•Fall speed: to 80 cm/s
•Fastest Growth rate: -14.7 C
a axes
basal face
Unit cells of Ice 1h
1022 units/cm3
d=2.76 A
c
a
projection view
Experimental Features
•Quadrupole levitation for wall-less environment
•Alternate support on fine glass fiber
•Vertical flow between 0 and 75 cm/s
•Temperature between room and -65 ºC
•Total pressure between atmospheric and 50 hPa
•Supersaturation controlled between 0 and greater than water saturation
Where In The World ?
Unique physical properties of water:
•Very high specific heat capacity
•High thermal conductivity for a liquid
•Neutral pH
•Exists on earth in 3 phases
•Ice is less dense than liquid
•Nearly universal chemical solvent
•High surface tension
•Large latent heats (energy of phase change)
•Needed as ingredient for beer
•In many instances,
multiple phase changes are
occurring simultaneously
at one surface.
•Temperature, pressure,
and humidity conditions
determine which
microscopic process
dominates
•The macroscopic behavior
is the net result of all
processes. We often are
concerned only with
explaining this net result.
•For enthalpy to increase,
heat must be absorbed
from surroundings. When
this energy is removed
from the surroundings, it
can be used to break
hydrogen bonds. This
causes the phase change
and cools the surroundings
due to heat loss.
•When enthalpy decreases,
heat is released to the
surroundings due to the
formation of new
hydrogen bonds. This
warms the surroundings
•The water substance has
constant temperature
during a phase change
Phase Diagram for Water:
Thermodynamic equilibrium
Clausius-Clapeyron Curve
e e0 exp
e e0 exp
Lv 1
Rv T0
1
T
Lv 1
Rv T0
1
T
•What is the difference between actual and saturation vapor
pressure?
•What is steam?
•What conditions must be met to allow boiling to occur?
•How will the boiling temperature change at high altitude?
Thermodynamics of phase changes in water:
•(blue = heat released to surroundings) – condensational warming
•(red = heat added (removed) from surroundings) – evaporative cooling
•Vaporization/Condensation:
Lv=2.27x106 J/kg
•Freezing/Melting:
Lf=3.3x105 J/kg
•“Latent Heat” is a measure of enthalpy change associated with hydrogen
bonds forming or breaking during a phase change.
•Expressions for heating
•How long must I microwave the frozen milk?
•Clouds are classified mainly by their visual characteristics and height
•They look different because they have different contents
•3 primary types and many sub-types
Stratus
Cumulus
Cirrus
Stratus Clouds
Characteristics:
•Can be at any altitude – stratus just means that they form a horizontal layer
•They are often at low altitude in bad weather (nimbostratus)
•Fog is a stratus cloud hugging the ground
•They are formed by weak, but widespread vertical motion (~10 cm/s)
•The are made of a moderate density of cloud drops , LWC~.1 g/m3
•Cumulus or cirrus can also form a layer (Stratocumulus and cirrostratus)
Cumulus Clouds
Characteristics:
•Can be at any altitude – cumulus means “heaping”
•They develop more vertically than horizontally.
•When they form rain they become cumulonimbus
•They are formed by strong vertical motion, sometimes 25 m/s updrafts
•Strong vertical motion and cumulus clouds result from free convection
that comes from instability
•If that vertical motion is deep enough, ice can form in upper part of the cloud
•Ice crystals and strong motion -> charge separation ->lightning
•They have the greatest LWC: from .5 to 4 g/m3 depending of updraft rate
Cirrus Clouds
Characteristics:
•Are composed of tiny ice crystals, not liquid cloud drops
•Usually form only when T< -25 C
•They are formed by weak vertical motion (~5 cm/s)
•The are made of a small density of ice crystals , IWC~.05 g/m3
•Sometimes generated by jet exhaust (contrail)
•Often initiated as anvils of cumulus clouds striking the tropopause-lid
•Important effects due to widespread radiative impact
Cloud Height
Cloud height
Cloud types
Low (below 2 km, 6500 ft)
Fog
Stratus
Nimbostratus
Stratocumulus
Stratus fractus
Cumulus humulis
Mammatus
Funnel
Middle(2-6 km, 6500-20000ft)
Cumulus humulis
Cumulus mediocris
Stratocumulus
Altostratus
Altocumulus
High (6+ km, 20000 ft+ )
Cirrus
Cirrostratus
Cirrus uncinus/fibratus
Pileus cloud
Large vertical span
Cumulus castellanus
Cumulus congestus
Cumulonimbus
http://apollo.lsc.vsc.edu/classes/met130/notes
/chapter5/graphics/drop_cloud_ccn.gif
CCN: Cloud Condensation Nuclei
•Needed to turn supersaturation into liquid drops (a site is needed for condensation)
•This is referred to as “drop nucleation” – a big uncertainty in the science of clouds
•CCN are preferentially hydrophillic
•Can be composed of dust, bacteria, pollen, pollutants, acid drops, salt, and others
•Ice nuclei have slightly different characteristics
Vapor deposition into water drops
m=
“net” molecules added
molecules impinging - molecules vaporizing
Will make it in
Made it out
Will be rejected
Cloud Drop
~20 um radius
m=
-
+
-
Vapor deposition onto ice surfaces
•Average coefficients help determine net mass growth rate
•Relative local coefficients determine habit type
•Local coefficient is a function of temperature and moisture density
•Mechanism for coefficient temperature function is an enduring mystery
•Mechanism of incorporating incident molecule into lattice is also unknown
On the way out
Cleared for landing
(condensation)
Brief layover
Taxiing to lattice
(surface diffusion)
m=
Taking off
(sublimation)
Actively growing
terrace
2D Nucleation
+
-
Bergeron-Findeison Process
•The saturation vapor over water is greater than ice (see phase diagram)
•This is caused by the greater difficulty in breaking 100% vs 80% of H-bonds
•Vapor tries to move from high concentration -> low concentration (law of diffusion)
•Thus, when water and ice surfaces are nearby, the vapor moves from high concentration
(water surface)->lower concentration (ice surface), allowing ice to grow as water evaporates
•This is the major form of ice crystal growth in mixed-phase clouds
•This process contributes to many stages of the precip. process
Little drops->Big drops: Collision-Coalescence
Riming->graupel->Hail
Frozen precip.
scenarios
Crystals
photographed in
cirrus clouds by
aircraft-borne
probe
Weather Modification and cloud seeding
Witches concoct a brew to summon a hailstorm.
“After the bomb, Dad came up with ice” – Kurt Vonnegut, Cat’s
Cradle, on the invention of “ice-nine”.